One problem which presents itself with measurements in the medium to long wavelength infrared is the radiation emanating from the various components of the instrument themselves. An essential contribution to such radiation is due to radiation from the filters themselves. This radiation is also modulated with the filter alternation frequency. It is not restricted to the narrow transmission range of the filters, and as the detector generally responds non-specifically to the total radiation, this spurious signal caused by the radiation of the filters themselves can result in considerable falsification of the measuring signal.
It is an object of the invention to provide a bi-frequency infrared spectrometer wherein the influence of the radiation of the filters themselves is eliminated.
A further object of the invention is to provide a signal evaluation circuit which permits clear separation of alternating measured quantities, the waveforms of which overlap partially.
A further object of the invention is to provide a signal evaluation circuit for a measuring instrument operating in accordance with the bi-frequency method to determine the concentration of a substance in a sample from extinction such that, on one hand, the influence of the radiation and of the different transmissivities of the filters can be compensated for, and, on the other hand, an output signal, which is a linear function of the concentration of the sought substance in the sample is obtained even with deviation from Beer's Law.
In one embodiment of the invention the filters are attached to a carrier so as to face the detector. The carrier has an aperture in the area of each filter. The dimensions of these apertures are smaller than those of the filters. The carrier is arranged to pass with its apertures through the path of the beam of radiation. The beam of radiation alternatingly falls through one and the other aperture and filter onto the detector and is covered, prior to the passage of the beam through an aperture, by the carrier arranged on the side facing the radiation source, while portions of the filter beside the aperture are in said path. A clamp circuit is provided, which is connected to clamp the detector signal to a baseline during the reference window and during the measuring window until shortly before the reference time interval and the measuring time interval, respectively, begin.
With such an arrangement, the detector "sees", at first, the filter surface, which is shielded by the carrier from the radiation source. Therefore the detector signal represents the radiation from the filter itself. This detector signal is clamped to a baseline by the clamp circuit. During the subsequent reference or measuring time intervals, only the variation of the detector signal relative to the detector signal caused by the radiation of the filter will be integrated. Thus a measured value independent of this filter radiation will be obtained.
According to another aspect of the invention, the output signal from the clamp circuit is applied to the integrator directly through a first switch controlled by the programmer, and inverted through a second switch also controlled by the programmer. One switch is closed by the programmer during each reference and measuring time interval, and the other switch is closed during compensation time intervals defined before each reference and measuring time interval. The clamp circuit is arranged to be actuated by the programmer to clamp the detector signal to a baseline before each reference, measuring and compensation time interval. The programmer is arranged to apply the output signal of the integrator or a function, such as the logarithm, of this signal to one memory circuit each, after each reference time interval and after each measuring time interval. The integrator is arranged to be reset to zero by the programmer after each application of the output signal to one of the memory circuits.
The programmer defines a time interval during which one measuring quantity is predominate, and a second time interval, in which the other measuring quantity is predominate. Herein these two time intervals will be called "reference window" and "measuring window". Each measuring quantity is measured by integrating the signal during a reference time interval, which is located within the reference window, and during a measuring time interval, which is located within the measuring window. In the reference window, however, there will be signal components which result from the measuring quantity measured within the measuring window and vice versa. For this reason, prior to the measurement of one of the measuring quantities by integration during the reference time interval, the signal is integrated during a compensation time interval. There is also an integration during a compensation time interval before the measuring time interval. Before each integration interval the detector signal is "clamped" to a baseline by a clamp circuit. The shape of the signal waveform at the detector with a predetermined, for example triangular, waveform of the measuring quantities has always the same character independent of the amplitudes of the measuring quantities. Therefore the integral of the detector signal formed during the compensation time interval has a fixed ratio to the integral, formed during the measuring or reference time interval, of the signal component resulting from the respective other measuring quantity. When, therefore, the gain of application of the detector signal during the compensation time interval and the length of the compensation time interval are selected properly, the influence of one measuring quantity on the measurement of the respective other one and vice versa is eliminated by the integrations carried out with opposite signs.
The signal waveforms at the detector with, for example, triangular waveform of the measuring quantity acting on the detector are rather irregular with conventional detectors, and are different from detector to detector with the same character. Therefore the application of the signals to the integrator during the various time intervals has to be adjusted individually for each detector.
According to a further modification of the invention, provision is made that a filter is connected to receive the output signal of the detector, the transfer function of said filter being inverse to the transfer function of the detector.
Also with such a filter alone separation of the signals might be achieved. If such a filter were used alone, accurate matching of the filter to the characteristics of the respective detector would also be required.
It has, however, been found that with the combination of the circuit described in the beginning with a series connected filter the transfer function of which is inverse to the transfer function of the detector, practicially no adjustment for matching to the specific parameters of the detector is required.
According to a further aspect of the invention an adjustable corrective voltage is superposed on the detector signal at the input of the integrator during the reference time interval and during the measuring time interval. The detector signal is applied to the input of the integrator through two different resistances controlled by a controlled switch. At least one of the above mentioned resistances is adjustable. The controlled switch is controlled by the programmer such that one resistance is effective during the reference time interval, and the other resistance is effective during the measuring time interval. The corrective voltages and the adjustable resistance are adjusted such that a substantially linear relation exists between output signal and concentration taking into account the radiation of components of the instrument itself and the different transmissivities of the filter as well as deviations of the extinction from Beer's Law.
By superposing the adjustable corrective voltages to the reference and measuring signals at the input of the integrator, curvature of the extinction-concentration characteristic can be compensated for, which curvature occurs deviating from Beer's Law. An offset of the zero point is compensated for by varying the ratio of the signals applied from the detector to the input of the integrator. Such variation of the ratio by a constant factor results in a term of a sum after formation of the logarithm, and this term can be selected to ensure that the output signal as function of concentration passes through the coordinate origin, thus the output signal becomes zero, when the concentration is zero.
The same corrective voltages may serve to compensate for the influence of the radiation of components of the instrument itself. The linearization described hereinbefore is achieved by "miss-compensation" of these instrument radiation influences.
In similar manner the adjustment of the adjustable resistance through which one of the signals is applied to the integrator may be used to take the "filter factor" into account. Also here a "miss-compensation" can ensure that the linearized concentration-output signal characteristic passes through the coordinate origin.
An embodiment of the invention is described in greater detail hereinbelow with reference to the accompanying drawings.